but a harem ending wouldn't really be an open ending if he chooses all of them because then it would actually be a conclusion

True I suppose... but I would rather see and open ending/harem ending with Oreshura than choosing any single girl. Loved all of them in the series, which is a rare thing to happen for me, usually hate one or two of them.

how is 13 an odd number to end on? 52 weeks /4 seasons is 13 episodes a season. it left it open for continuation and if it sells well they'll probably come out with a season 2 if not as well they will probably go with ovas. either way harem series almost always have crappy endings because:
1 the animators dont want to offput any sales by picking a hero/ine that fans dont like.
2 picking a hero/ine literally ends the series.